Increasing the capacity of optical communications systems is required in order to satisfy the increasing demand for data traffic and to reduce the cost of data transmission. Among the ways to achieve this, serial to parallel conversion and the increase of the symbol rate are widely used.
Serial to parallel conversion enables to split a serial high bit rate channel into lower bit rate channels. The lower bit rate channels are transmitted as parallel channels between the same start point and the same stop point of the transmission, and the parallel channels do not have any crosstalk, ideally. After reception, the parallel channels are serialized into a higher bit rate channel, which contains the transmitted information of the original serial channel. This enables to decrease the effect of the distortions and impairments appearing during the transmission because the effects increase with the symbol rate. It also enables to use more transmission channels to transmit a huge amount of data, which could not be transmitted on a single channel, and enables to handle them logically as a single channel.
Known ways to implement parallel transmissions include: Polarization Division Multiplexing (PDM), where two signals are transmitted through the same medium by two signals having orthogonal polarizations; Wavelength Division Multiplexing (WDM), where the parallel channels are transmitted through the same medium by different optical carriers having different wavelengths; Space Division Multiplexing (SDM), where the parallel channels are transmitted through different mediums which can be bounded such as a fiber ribbon; and combinations of the previous multiplexing formats.
When using PDM, skew can appear between the multiplexed signals, for instance due to Differential Group Delay (DGD) which changes randomly on the transmission line. According to the encountered DGD, the skew between polarizations can exceed half a symbol period. In this case, when the data of the multiplexed polarizations are re-serialized after reception, the order of the recovered data bits is altered and the decoding of the received information is no longer possible.
Parallel transmission can also be implemented with WDM. The parallel lower rate channels are transmitted by lightwave carriers at different wavelengths or frequencies through the same medium. At the other end of the link, the parallel signals are demultiplexed according to their wavelengths; each channel is received and decoded by receivers. The electrical data from the parallel receivers are then converted from parallel to serial. Intra channel skew can appear inside the medium due to chromatic dispersion (CD) or due to the difference of length between the electrical paths after reception.
Another way to implement parallel transmission is SDM which uses fiber ribbons. The link skew in SDM is due to difference in the fiber length or conditions. With higher bit rates, the symbol rate transmitted through the fiber increases, which means the symbol periods become shorter. Therefore, the skew problem becomes critical with higher bit rates.
Therefore, increasing the transmission capacity through parallelization requires a way to compensate the skew between the parallel channels after the reception and before the re-serialization. This can be done by adding more information to the channel in addition to the data, in order to get information on the skew between the parallel channels. This may be done on the physical layer or on higher layers.
On the other hand, increasing the symbol rate or baud rate of the transmitted signal enables to increase the capacity of a transmission channel and therefore the total capacity of the transmission system. However, impairment appearing inside the transmission medium, which is related to physical constant of the medium such as DGD or CD, has a bigger impact on signals with higher bit rates. In other words, signals with higher baud rates are more affected by CD or DGD. As a consequence, means of monitoring CD and DGD, which relate to the signal baud rate, will have tighter monitoring ranges when they are applied to higher baud rate signals. Monitoring impairments can be done by adding more information to the channel in addition to the data on the physical layer or higher layers.
Adding information to the signal in order to monitor impairments or skew between parallel channels can be done in different ways. One can be grouped as data aided methods.
An example of related data aided methods is disclosed in the non patent literature 1 (NPL1). In data aided equalization, training patterns are used to eliminate the ambiguity on polarizations and transmitted symbols. All possibilities for the attribution of ambiguous variable are tried until the training pattern is recognized. In this case, buffer or time delaying filters can be used to correct the delay between polarizations until the training pattern is recognized. Alternatively, the received pattern can be compared with several recognition patterns for possible cases of delay until there is a match, which enables to retrieve and correct the delay between polarizations.
In a similar manner, non patent literature 2 (NPL2) discloses that the skew between channels in WDM is rectified by using of the XAUI standard, which relies on channel decoding and realigning. This requires an increase of the total bit rate for the same transmitted data payload because the prefix needed for alignment is introduced in the transmitted data. In addition, the skew monitoring requires the information of upper layer information.
In conjunction with the above description, patent literatures 1-4 (PTL1-4) disclose that the skew between parallel channels can be monitored and corrected.
According to PTL1, a special pattern is required which is named PING or PONG depending on the case, and is added to the actual data to be transmitted. PING and PONG pattern carry no information and are used only for the purpose of skew monitoring or compensation. Therefore, to carry effectively m bits of data within the network, it is necessary that a number of bits contained in PING or PONG are added to m bits to be physically transmitted through the network. Moreover, in PTL1, the receiver must distinguish PING and PONG patterns from the data.
According to PTL2 as well as PTL1, the synchronization overheads are inserted into the transmitted data and are received for different parallel channels to monitor skew.
Also, according to PTL3, it is necessary that m′ frame bits are added to the actual data to be transmitted. Therefore, to carry effectively m bits of data within the network, m+m′ bits are required in fact to be physically transmitted through the network.
According to PTL4, the multiplexed optical-packet signal is transmitted to a transmission destination and it is retransmitted back to a transmission source. At the head of each packet, there is a skew detection byte for detecting a skew amount. A skew amount of the optical packet is detected by comparing the arrival timing of the skew detection byte of the packet with that of the first arrival packet as a reference.
Adding information to the signal in order to monitor impairments or skew between parallel channels can be done in another way. This can be grouped as tone aided methods. The information on skew between parallel channels is necessary to compensate the skew. The information could be obtained by imprinting a tone signal at the transmitters of different channels and by extracting the tone at the receiver and comparing the phase of the extracted tones. Identically, the tone aided methods enable to monitor other impairments such as chromatic dispersions.
The easiest implementation is to use in-band tones, where the tone frequency is lower than the baud rate of the transmitted signal. Implementations and problems related to in-band tone signals are illustrated in the non patent literature 3 (NPL3). Low frequency tones, namely in the kilohertz range, can be easily achieved by directly modulating the laser.
In a similar manner, the non patent literature 4 (NPL4) disclosed the use of an in-band pilot tone in order to monitor CD impairing the optical signal, on which the tones are imprinted.
Higher tone frequencies can be achieved at the cost of moderate complexity by imprinting the tone on the modulator modulating the optical signal. In this way, better monitoring precision could be achieved, but the method is limited by the inherent sensitivity degradation as the tone is imprinted in-band with the signals, and the signal symbols are affected by the tone modulation. Moreover, the realization of reliable skew monitors requires tones imprinted with high modulation indices, which increase the signal degradation in the process.
The non patent literature 5 (NPL5) showed that the penalty caused by in-band tones could be reduced by using out-of-band tones. In that method, the tones are up-converted with a subcarrier at the frequency of twice the baud rate of the signal to reduce the degradations due to tone signal interaction when using practical modulation indices for skew monitoring.
Another example of a method enabling monitoring transmitted signals is disclosed in the non patent literature 6 (NPL6). In NPL6, it is proposed to use Pseudo-Return-to-Zero modulation scheme in order to monitor the skew between parallel channels. The retrieved signals after reception contain the information of the skew between polarization multiplexed channels. The information could be used to compensate for the skew.